In recent years, a radio communication apparatus that makes use of a wireless LAN (Local Area Network) is mounted on a vehicle-mounted apparatus. On the other hand, smartphones and tablet terminals are widely spread and a large number of pedestrians own radio communication apparatuses. Therefore, it is under consideration to perform danger prediction making use of radio communication between the radio communication apparatus of the vehicle-mounted apparatus and the radio communication apparatuses of the pedestrians (inter-pedestrian-vehicle communication or inter-road-vehicle communication). In the radio communication between the radio communication apparatuses, a usable frequency channel is probed, that is, searched to find a communication partner terminal and, if the communication partner terminal is communicable, communication is started.
As means for finding a communication partner in an infrastructure mode of IEEE (the Institute of Electrical and Electronics Engineers, Inc.) 802.11a/b/g/n/ac in a wireless LAN, active scanning and passive scanning have been present.
In the active scanning, the radio communication apparatus starts the active scanning as soon as the radio communication apparatus receives an “MLME-SCAN. request” primitive, a “ScanType” of which is active scan.
Note that the “MLME-SCAN. request” primitive refers to a primitive, that is, a command generated by an SME (Station Management Entity) when the radio communication apparatus searches for a communication partner and notified to an MLME (MAC subLayer Management Entity).
In the active scanning, the radio communication apparatus stays on standby until a probe delay time, which is a fixed time, elapses in a certain frequency channel or the radio communication apparatus receives a beacon frame transmitted from an access point. When the radio communication apparatus does not receive the beacon frame and the probe delay time, which is the fixed time, elapses, the radio communication apparatus starts a broadcast of a probe request frame for finding a communication partner. After starting transmission of a probe request, the radio communication apparatus starts a probe timer and determines whether a “PHY-CCA. indication (Busy)” is passed from a physical layer to a MAC (Medium Access Control) layer within a “MinChannelTime”, which is a fixed time after the start.
Note that the “PHY-CCA. indication (Busy)” is a primitive, that is, a command for notifying from the physical layer to the MAC layer that a frequency channel is used by another radio communication apparatus.
When the “PHY-CCA. indication (Busy)” is not notified from the physical layer to the MAC layer within the “MinChannelTime”, the radio communication apparatus starts a scan in the next frequency channel.
On the other hand, when the “PHY-CCA. indication (Busy)” is notified from the physical layer within the “MinChannelTime”, after the elapse of a “MaxChannelTime” from the start of the probe timer, the radio communication apparatus performs processing of received all probe responses and thereafter starts a scan in the next frequency channel.
Note that the “MinChannelTime” is a minimum time consumed in frequency channels in the scan. The “MaxChannelTime” is a maximum time consumed in the frequency channels in the scan. The “MaxChannelTime” is defined as a value equal to or larger than the “MinChannelTime”. The operation described above is performed in frequency channels designated by a “ChannelList”.
A radio communication apparatus that searches for a communication partner is represented as a terminal STA1 and a communication partner radio communication apparatus is represented as a terminal AP1. The terminal STA1 searching for a communication partner starts a broadcast of a probe request frame after determining a frequency channel to be scanned.
The terminal AP1, which receives the probe request frame, transmits a probe response frame to the terminal STA1 after the elapse of a DIFS (DCF (Distributed Coordination Function) InterFrame Space) time. The terminal STA1, which receives the probe response frame, transmits an ACK (acknowledgement) frame after the elapse of an SIFS (Short InterFrame Space) time. The terminal STA1 starts processing of received all probe responses after the elapse of the “MaxChannelTime” from the start of the transmission of the probe request. The terminal STA1 sets a “ScanType” to start the active scanning or the passive scanning. The STA1 sets time until the transmission of the probe request according to the probe delay time. A value of the “MinChannelTime” and a value of the “MaxChannelTime” are also set to fixed values according to the “MinChannelTime” and the “MaxChannelTime”.
Concerning the frequency channel to be scanned, a physical frequency list usable by terminals is set according to the “ChannelList”. When receiving the probe response frame, the physical layer of the radio communication apparatus notifies the MAC layer of the “PHY-CCA. indication (Busy)”. The presence of the “PHY-CCA. Indication (Busy)” means that the probe response frame is received from another radio communication apparatus and means that there is another radio communication apparatus currently using the frequency channel and it is likely that the radio communication apparatus can communicate with the other radio communication apparatus. The absence of the “PHY-CCA. indication (Busy)” means that another radio communication apparatus is absent.
In the above explanation, if the “PHY-CCA. indication (Busy)” is present, it is assumed that a candidate of a communication partner is present in the frequency channel. If the “PHY-CCA. indication (Busy)” is absent, a candidate of a communication partner is absent in the frequency channel.
In the passive scanning, the radio communication apparatus starts the passive scanning as soon as the radio communication apparatus receives a “MLME-SCAN. request” primitive, a “ScanType” of which is a passive scan. The radio communication apparatus starting the passive scanning waits for beacon frames from frequency channels designated by the “ChannelList” and waits for beacon frames in the next channel after the elapse of the “MaxChannelTime”.
As an example, Patent Literature 1 discloses a technology in which, in direct communication between radio communication apparatuses not via an access point, terminals of the radio communication apparatuses set the order of a scan of frequency channels at random, determine, at random, scanning start time in scanning the frequency channels, determine the “MaxChannelTime” and the “MinChannelTime” at random in a range in which the “MaxChannelTime” and the “MinChannelTime” respectively do not exceed a maximum of the “ChannelTime” and a maximum of the “MinChannelTime”, and perform a sequential scan to detect the terminals early.